Aircraft: B757
Phase of flight: Cruise
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy

In cruise flight FL3XY; we received a yellow EICAS Left Hydraulic Quantity (L HYD QTY) message. We opened the QRH and followed the checklist for L HYD QTY message which had us turn off the associated Left hydraulic pumps in order to conserve (if able) quantity in the system. We had the status page up and noticed it was approximate .47 on the Quantity listed; and then also noticed it continued to slowly drop in number as we continued to complete the checklist. The checklist had us reference Hydraulic System Pressure -Left (only) checklist to see affected subsystems (extensive list and checklist). When referencing this and seeing the involvement with degraded flight control systems (spoilers; nose wheel steering; thrust reversers; etc.) and looking at ZZZ's current and forecast wind (strong steady and gusty crosswinds); Captain transferred controls and ATC communication to me and began consultation with Company on best options for us. The quantity during his conversation with Company continued to drop and I (with his agreement) slowed the cruise speed in case we elected to divert based off of possible flight control degradation based on the conditions in ZZZ. We both discussed the nice conditions back in ZZZ1 and long runway for a potential flaps 20 landing with other degraded flight control systems. As the discussion with Dispatch/Maintenance Control/ Chief Pilot continued; the hydraulic quantity continued to drop and then stabilized around .17 (where the standpipe sits in the reservoir?). Hydraulic pressure continued steady at approximately XY00 with some slight fluctuations and we agreed (temporarily) that ZZZ would still be viable based off of the Power Transfer Unit (PTU) input to the Left Hydraulic system.As the conversation with the Company continued; the Hydraulic pressure then started to fluctuate and eventually went to zero along with the quantity. At that moment; the status message POWER TRANSFER UNIT popped up. The captain relayed to Company this information and with the concurrence of all; the election to divert back to ZZZ1 was made based off of all the available information of system degradation; weather conditions in ZZZ and time to complete the extensive HYDRAULIC SYSESTEM PRESSURE - Left (Only) checklist. With concurrence; I advised ATC our wishes to divert to ZZZ1 and [request] for priority handling. Shortly after receiving revised clearance; ATC coordination and required information for them; we reprogrammed the box and began our diversion back to ZZZ1. Within a few minutes; we received an ACARS message from Dispatch that ZZZ2 was reporting good conditions with a favorable runway alignment for the winds. Captain and I evaluated runway XY (RWY XY) and agreed ZZZ2 was a good option at only 233NM from our current position (plenty of time to reprogram; complete the involved checklist and land safely with a direct headwind on RWY XY).At the moment of agreement; we then asked for diversion to ZZZ2 for RWY XY under emergency authority with full fire equipment standing by for precautionary purposes. We were then given revised clearance to ZZZ2 and completed the flight without further incident after completing the checklist.Being that the nose wheel steering was inoperative; we stopped the aircraft on the runway after landing on the centerline and waited for evaluation by fire/rescue and awaited tow-in to gate from the runway to the gate from a supertug. Supertug tow was delayed for several minutes after evaluation from fire/rescue and their report of significant fluid leakage from the aircraft right wheel well pertaining to the system malfunction. B757 Captain reported having to divert after losing yellow system hydraulic quantity during cruise.